1. Field of the Invention
This invention relates to a current switching circuit having at least one current switch and a constant current source.
2. Description of the Related Art
FIG. 4 shows an example of the conventional current switching circuit. The current switching circuit includes current switch 2 having bipolar transistors Q1 and Q2 whose emitters are connected to each other and current source 3 having bias voltage source VB, resistor R1 and bipolar transistor Q3 which is set in the conductive state. A logic input (IN, IN) is supplied to the base of transistor Q1 and a reference voltage (logic threshold adjust) is supplied to the base of transistor Q2. Further, the collector of transistor Q1 is connected to output circuit 1 and the collector of transistor Q2 is connected to power source VDD.
The current switching circuit controls the current by selectively turning on one of transistors Q1 and Q2 according to the logic input supplied to the base of transistor Q1.
Assume that the output of a CMOS inverter is used to drive current switch 2 of the above conventional current switching circuit. In a case where the bipolar transistor is directly driven by the output of the CMOS inverter, there is a possibility that the bipolar transistor will be operated in a saturation region. Therefore, it is necessary to sue a level converting circuit so as to convert the output level of the CMOS inverter to a corresponding level within the operation range of the bipolar transistor. Use of the level converting circuit makes the circuit construction complex. In contrast, where current switch 2 is formed of FETs, the FET can be driven by the output of the CMOS inverter without using a level converting circuit. However, in this case, the characteristic of current source 3 becomes deteriorated. This is because drain current ID will greatly vary with respect to voltage VDS between the source and drain because of the channel modulation of the FET. Even when the Early effect of the bipolar transistor is taken into consideration, the bipolar transistor type current source can supply a more stable current.